Several ink compositions containing a cationic, water-soluble polymer have been proposed as ink compositions for yielding waterfast prints.
For example, Japanese Patent Laid-Open No. 119280/1987 discloses an ink comprising a hydroxyethylated polyethyleneimine polymer and a dye component. This publication describes that polyethyleneimine, when used either alone or in combination with an acid or reactive dye, can develop waterfastness in the printed ink. Japanese Patent Laid-Open Nos. 255876/1990, 296878/1990, and 188174/1991 disclose an ink composition comprising a polyamine, with a molecular weight of not less than 300, having a primary amino group, an anionic dye, and a stability-imparting agent, such as urea or thiourea. These publications describe that the combination of the primary amine with the anionic dye can develop waterfastness in the printed ink.
Japanese Patent Laid-Open No. 305011/1995 discloses a water-base ink comprising a basic, water-soluble polymer, an anionic dye with the counter ion being a volatile base, and a buffering agent with the counter ion being a volatile base. This publication describes that, in the ink, the volatile base prevents the dissociation of the polymer and, after printing, the volatile base on paper is evaporated to allow a salt-forming reaction between the polymer and the dye to proceed, thereby developing waterfastness.
In the ink compositions comprising an anionic dye and a basic resin, the development of the waterfastness is considered attributable to a reaction of an anion of the dye with a cation of the resin on a recording medium. This is probably because, although both the dye and the resin are ionized and dissolved in the ink with a satisfactory amount of water being present therein, a reduction in amount of water as a result of drying to some extent after printing permits both the dye and the resin to be precipitated as a water-insoluble salt, thus developing the waterfastness.
These ink compositions, when used in ink jet recording under conventional conditions, do not pose any clogging problem. However, when they are allowed to stand under such conditions as will extremely accelerate drying, for example, in high temperature and low humidity environment, for a long period of time, the ink is often dried and solidified at the front end of nozzles, making it impossible to eject the ink. For this reason, an ink composition which does not cause the nozzle clogging problem even under such conditions has been desired in the art.
Japanese Patent Laid-Open No. 8472/1981 discloses an ink comprising an acid dye or a direct dye and a pyridine derivative. The pyridine derivative is added in order to improve the color development of the dye and to prevent an increase in viscosity of the ink. In this ink, however, waterfastness could not have been realized.